Neurophysics

The Neurophysics group is headed by Assoc. Prof. Axel Thielscher and situated both at DRCMR and DTU Elektro. Our main focus is on advancing non-invasive transcranial brain stimulation (NTBS) methods as a means to modulate and shape brain activity. NTBS uses electric currents that are focally induced in superficial brain areas. We develop and apply biophysical models to reveal and optimize the current flow patterns in the brain and to estimate their impact on neural activity. The computational modeling work is complemented by applying neuroimaging approaches such as functional MRI (fMRI) and electroencephalography (EEG) to better characterize the impact of neurostimulation on brain activity. We are interested in human sensorimotor integration and motor control, which sets the neuroscientific scene in which we employ and test the NTBS methods.

Key projects

Interleaved or concurrent TMS-fMRI

The online combination of both techniques allows us to directly assess the effects of TMS on the blood-oxygen-level dependent signal (BOLD) measured by fMRI. By this, we can, e.g. look at the changes in effective connectivity between brain areas that accompany changes in the behavioral task or in sensory stimulation. For example, we compared the effects of left PMd stimulation on the activation patterns of two motor tasks, one relying on learned sensorimotor associations while the second allowed free response selections (Moisa et al, 2012). TMS applied to the left PMd exerted specific network effects only for the task relying on sensorimotor associations. Therefore, the TMS perturbation yielded causal evidence that the left PMd is implicated in mapping external cues onto the appropriate movement in humans. This confirms the results of prior primate studies. Currently, Sofie Johanna Nilsson is a main contributor to this project.

SimNIBS is both a scientific and an open-source software development project (Thielscher et al. 2011). We use field calculations to estimate the current flow generated by NTBS methods in realistic head models which are reconstructed from structural MR images. This allows us to determine the likely stimulated brain areas, and in turn to optimize the stimulation patterns. After developing the methodology, we are currently focusing on testing how well the estimated fields correlate with the experimentally observed stimulation effects. Simultaneously, we are strongly extending SimNIBS to include optimization methods for multi-electrode current steering for transcranial weak current stimulation (TCS), and to enable forward calculations for EEG and MEG in highly realistic head models. Currently, Oula Puonti, Guilherme Saturnino, Jesper Duemose Nielsen and Peter Jagd Sørensen are main contributors to this project.

Advanced acquisition methods for MREIT and MRCDI

Magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance current density imaging (MRCDI) are evolving as methods to directly measure the ohmic conductivity distribution in the head and the current flow pattern generated by transcranial weak current stimulation (TCS). They would be tremendously useful to validate the field estimates from computational models, or might be used as alternative to them giving similar information. Their main drawback is the limited sensitivity of the MR measurement sequences which prevent their usage in human applications. We are currently developing optimized MR sequences, with the aim to enable MREIT measurements of the human head. Cihan Göksu is a main contributor to this project

Transcranial focused weak ultrasound stimulation (tFUS)

Basic physics sets limitations on the spatial resolution which can be reached by classical NTBS methods and prevents the targeted stimulation of deeper brain areas. TFUS is emerging as an alternative which does not suffer from these principal limitations. However, TFUS is at a very early stage of development. We are excited about our newly started project in which we aim at implementing TFUS for human brain stimulation. Cristina Pasquinelli is a main contributor to this project.

The EXMAD project is driven by our collaborators Ulrik Lund Andersen and Alexander Huck (DTU Physics) to enable highly sensitive measurements of the magnetic fields of neurons. We provide highly realistic simulations of the neuronal magnetic fields to guide methods development. Mürsel Karadas is a main contributor to this project.

In addition to our core projects, we contribute to several projects driven by external partners. We would like to highlight the simulations of Tumor treating Treating fields (TTFs) in collaboration with Anders Korshøj (Aarhus University) as example. In that project, we apply the methodology developed in SimNIBS to estimate the electric field created by TTF in the braintumor treating fields (TTF).